Small cylinders containing carbon dioxide are well known and are available under the registered trade marks sparklets and sodastream. Such cylinders normally have capacities between 300 and 405 cubic centimeters and are normally used to supply gaseous carbon dioxide for domestic water carbonators and, more recently, to dispense either gaseous or liquid carbon dioxide for use in pneumatic power devices, such as model aircraft motors, power tools, garden pressure sprayers, automatic shavers, automatic starters for petro-engined lawn mowers, wherein the high-pressure carbon dioxide provides mechanical power. A preferred embodiment of the present invention, described later in this specification, is intended for suchlike uses especially.
However, the present invention may also be employed in a wide variety of other applications, such as fire extinguishers, cylinders containing medical gases, cylinders containing a variety of gases or liquids as already distributed for use in laboratories, and much larger (e.g. 5-50 liter capacity) cylinders, such as those used in the distribution of nitrogen, oxygen, propane, butane, carbon dioxide and acetylene, to industrial users for welding, metal-cutting, heating and other uses.
The background art employed in these known types of cylinders suffers from a number of disadvantages. For example, the most favored method of constructing cylinders for use in carbonators and fire extinguishers employs steel tubing which must be heated (usually by a gas flame) until the steel can begin to flow, whereupon the base of the cylinder is closed by hot-spinning; this process often causes slag-inclusion in the base, weakening the base and allowing slow leakage of gas from the cylinder during service. In addition, the other end of this type of steel cylinder (to which the valve is affixed) is also formed by hot-spinning or hot-swaging s as to provide a "neck reduction", and this process is usually labour-intensive and costly. Both of these hot-forming operations produces oxides which, despite subsequent interior washing, remain on the interior walls and then later become detached during service and so contaminate the cylinder contents. Furthermore, the steel commonly used in these cylinders is prone to corrosion by moisture and other contaminants present in many commercial gases. All such corrosion products and oxides tend to become finely divided and so can then pass through the filter usually provided in the cylinder's valve assembly, causing contamination of the carbonator or other appliance served by the gas cylinder.
In another known form of cylinder construction, the cylinder is formed by cold deep-drawing of steel sheet and wall-ironing, followed by neck reduction to permit attachment of the valve assembly. The extensive cold working of the steel during this process necessitates subsequent heat treatment of the entire cylinder, which removes much of the strength that had been imparted by the wall-ironing process.
Both the hot-forming of the first-mentioned method of cylinder construction, and the heat treatment needed in the second method, result in a considerable reduction in the strength of the steel wall and, in consequence, the wall thickness must be significantly increased for any desired burst pressure. Hence such cylinders are unduly wasteful of steel material, costly and heavy--which increases their cost of transportation.
Finally, in a third known type of construction the cylinder has one or two domed ends which are attached to a central cylindrical metal section by welding-on the domed ends and, sometimes, also has a welded seam in the cylindrical section. Such welding is costly and prone to defects and, furthermore, affects the material properties adjacent to each weld line so that subsequent heat treatment is often necessary, as well as giving rise to contamination of the cylinder interior.